The organic electroluminescence device comprises a substrate, a light transmissive electrode which acts as an anode, a hole transport layer, a light emissive layer, an electron transport layer, and a light reflective electrode which acts as a cathod. The light transmissive electrode, the hole transport layer, the light emissive layer, the electron transport layer, and the light reflective electrode are formed on the substrate in order. When the voltage is applied between the light transmissive electrode and the light reflective electrode, electrons are injected into the light emissive layer through the electron transport layer. At the same time, the hole is injected into the light emissive layer through the hole transport layer. The hole in the light emissive layer is recombined to the electrons in the light emission layer, whereby the light is generated in the light emissive layer. The light generated in the light emissive layer is emitted through the light transmissive electrode and the light transmissive substrate.
The organic electroluminescence device is configured to emit the light spontaneously. In addition, the organic electroluminescence device is configured to emit the light having a high efficiency. The organic electroluminescence device is configured to emit the light having a variety color hue. Therefore, the organic electroluminescence device is of interest in employing lighting elements for the displays such as flat panel displays and also in employing light sources for the liquid crystal display instruments and the lighting fixtures. In addition, a part of the organic electroluminescence device is commercially used.
The organic electroluminescence device is realized by a thin film device having a thickness equal to optical wavelength order. The thickness of the device has closely correlative relationship with respect to the luminescence property. Therefore, the organic electroluminescence device should be designed to have an optically suitable thickness and also an electrically suitable thickness.
In the organic electroluminescence device, the light generated in the light emissive layer is totally reflected by boundaries of the electrodes and the layers. As a result, the light is confined within the light emissive layer, the electrodes, and the substrates. According to a simple estimation, when the light emissive layer generates the light, about 50% of the light is confined within the light emissive layer and the electrodes. About 30% of the light which is generated by the light emissive layer is confined within the substrates. Therefore, only about 20% of the light which is generated by the light emissive layer is emitted from the organic electroluminescence device to an outside. On the other hand, the organic electroluminescence device having a light scattering region is proposed in order to increase an amount of the light emitted from the organic electroluminescence devices. This configuration makes it possible to increase an amount of the light emitted from the organic electroluminescence devices to the about 40%.
In order to increase an amount of the light emitted to the outside, it is important to exactly determine distance between a luminescent point in the light emissive layer and the light reflective electrodes. The distance between the luminescent point in the light emissive layer and the light reflective electrodes is determined on the basis of a phase shift caused by the boundary between the light reflective electrode and an adjacent layer which is adjacent to the light reflective electrode. For example, Patent literature 1 shows a relationship of the luminescent point, the light transmissive electrode, and the light reflective electrode. Patent literature 1 discloses the luminescent point in the light emissive layer which is spaced from the light transmissive electrode by a first distance. The first distance is approximately equal to even integer multiple of a quarter of a luminescence wavelength. Patent literature 1 also discloses the luminescent point in the light emissive layer which is spaced from the light reflective electrode by a second distance. The second distance is approximately equal to odd integer multiple of the quarter of the luminescence wavelength. In contrast, Patent literatures 2 and 3 disclose that the phase shift is determined on the basis of an optical refraction index and an extinction coefficient of the layers between the light transmissive electrode and the light reflection electrode. The distance between the light transmissive electrode and the light reflection electrode is determined on the basis of thus determined phase shift. Patent literatures 4 and 5 also disclose that the distance between the electrodes is determined to a predetermined value.
That is, these organic electroluminescence devices have luminescent coefficient which depends on the thickness of the light transmissive layer. Especially, when the layers between the luminescent point and the light reflective electrode have limited optical thicknesses, the organic electroluminescence device has a good luminescent coefficient. The limited optical thickness is exemplified by an optical wavelength equal to (2 m+1)/4 times of the luminous wavelength. (“m” is an integer more than zero.)
However, the patent literature 1 discloses that when the light is reflected by the boundary of the light reflective electrode and the adjacent layer which is adjacent to the light reflective electrode, the phase of the light is shifted by π. Therefore, the phase shift is sufficiently unconsidered by the patent literature 1. In addition, the patent literature 2 discloses a case where a half value width of a spectrum is limited to equal to or less than 50 nm. In addition, although the patent literature 3 discloses the distance between the light transmissive electrode and the light reflective electrode, the distance between the luminescent point and the light reflective electrode is not defined. Furthermore, the patent literatures 1 to 3 does not discloses the light scattering region provided to the components such as substrates.
On the other hand, the patent literatures 4 and 5 disclose the organic electroluminescence devices which include the light scattering region. The light scattering region is configured to reflect the light and also is configured to vary refraction angles irregularly. Each one of the electroluminescence devices in the patent literatures 4 and 5 has the optical thickness which is more than (m+4)/4 times larger than the luminous wavelength. It is noted that thus formula is applied when “m” is the integer which is equal more than 0. Thus designation causes a thickness reduction between the light emissive layer and the light reflective electrode, whereby the designation causing an electrical short. In addition, if the organic electroluminescence device has two light emissive layers, a certain degree of a distance between the light emissive layer and the light reflective electrode is required. Therefore, the organic electroluminescence device having two light emissive layers is not capable of employing the thickness designed according to the patent literatures 4 and 5. This problem is also included by the organic electroluminescence devices in the patent literature 2.
Therefore, the light emissive layer needs to be spaced from the light reflective electrode by the distance of a certain degree. However, an organic electroluminescence device having the luminescent point which is spaced from the light reflective electrode by an arbitrary distance which is suitably determined on the basis of an arbitrary photoluminescence spectrum and which is larger than a certain degree is not reported. Especially, design principle of the organic electroluminescence device having the light scattering region is not indicated.
In addition, an organic electroluminescence devices having a light transmissive electrode, an organic semiconductor, an inorganic semiconductor, a charge supply layer composed of such as a charge generation layer made of an electron acceptability material and an electron donor material. In the organic electroluminescence devices, the light transmissive electrode, the organic semiconductor, the inorganic semiconductor, and the charge supply layer are interposed between the light emissive layers which are stacked along a thickness direction of the organic electroluminescence device. The organic electroluminescence device of this type is reported to achieve a high brightness and a long operation life. Therefore, these organic electroluminescence devices are of interest of a future commercial use. However, an optical design principle of the organic electroluminescence device with respect to the future commercial use is unapparent. In addition, literatures on these organic electroluminescence devices report no designing to employ the light scattering region. For example, when the optical distance between the luminescent point and the light reflective electrode is designed to have the thickness approximately equal to the odd integer multiple of one-quarter of the luminous wavelength, there is not necessarily that the organic electroluminescence device emits the light which increases to approximately equal to multiple of the number of the light emissive layers. In addition, when the light scattering region is disposed on the substrate in this design, this design causes decrease of an amount of the light emitted from the organic electroluminescence device.    Patent Literature 1: Japanese patent application publication No. 2000-243573A    Patent Literature 2: Japanese patent application publication No. 2004-165154A    Patent Literature 3: Japanese patent application publication No. 2006-253015A    Patent Literature 4: Japanese patent application publication No. 2004-296423A    Patent Literature 5: Japanese patent application publication No. 2004-296429A